Automatic transmissions are configured to transfer drive torque from an IC engine to a drivetrain (e.g., wheels) via a particular gear ratio. For example, a six-speed automatic transmission defines six selectable gear ratios for the transfer of the drive torque. A greater number of speeds for an automatic transmission generally means that a larger gearing spread is achieved and that a higher number of gear shifts could be experienced by the driver. To mitigate noise/vibration/harshness (NVH) and to reach fast shift speeds for performance reasons, accurate and quick torque control (mainly torque reduction) of the IC engine is required. Three primary paths of engine torque control are spark retardation, fuel/cylinder shutoff, and airflow control (throttle valve, wastegate valve, etc.).
Spark retardation and fuel/cylinder shutoff typically decrease torque output of the engine faster than airflow control. Spark retardation, however, requires additional enrichment due to delayed combustion phasing at higher power levels, and is thereby limited, as well as having an overall combustion limit. Fuel/cylinder shutoff is only able to change output torque in fixed increments according to the ratio of firing versus motoring cylinders, e.g., one cylinder equals 25% torque reduction on a four cylinder engine. Additionally, fuel/cylinder shutoff causes an imbalance for the rotational speed.
These torque control paths are also sometimes inadequate for decreasing the engine torque output by a desired amount for such higher-speed automatic transmissions (e.g., 70% or more torque reduction), particularly when the engine is operating at a high speed/load. Spark retardation also causes increased exhaust gas temperatures, which potentially damages components (e.g., for turbocharged applications) if not mitigated by additional enrichment. Accordingly, while such transmission control systems work well for their intended purpose, there remains a need for improvement in the relevant art.